This invention generally relates to a transmitter/receiver and more particularly to an improvement in a frequency modulation (hereinafter referred to as FM) radio transmitter having a digital frequency synthesizer.
A voltage controlled oscillator (hereinafter referred to as VCO) serving as an oscillator of the frequency synthesizer in this type of FM transmitter has a non-linear output oscillation frequency characteristic with respect to the VCO control voltage applied to the control terminal of the VCO, owing to a non-linear characteristics of a voltage-dependent variable capacitor and other non-linear factors, though the output oscillation frequency characteristic should ideally be linear.
With the frequency synthesizer using a VCO having the above oscillation frequency characteristic as the oscillator, when FM modulation is effected by applying a modulation input signal in addition to the VCO control voltage, the modulation sensitivity level varies with the oscillation frequency of the VCO, though the modulation sensitivity level should ideally remain unchanged relative to the VCO oscillation frequency. In other words, when the center frequency of the VCO oscillation frequencies is changed to switch the transmission carrier wave frequency, the modulation sensitivity level is varied corresponding to the center frequency. If the relation between the modulation sensitivity level and VCO oscillation frequency is such that the modulation sensitivity level decreases as the VCO oscillation frequency increases, modulation will be deeper for low transmission carrier frequencies and shallower for high carrier frequencies even when the level of the low frequency signal applied as the modulation signal to the modulation signal input terminal of the VCO is fixed.